Figure 2: Recrystallization experiments probed by TEM.

The first row shows a recrystallization experiment on AgInSbTe probed by in situ TEM during annealing at 383 K. From electron diffraction measurements on a selected area within the mark (inside the red dotted circle), one can infer that the material is completely amorphous there (inset of section a). The phase transition occurs via crystal growth from the amorphous–crystal interface. The radius of the amorphous bit decreases in time, up to the complete erasure. The figure sections a, b and c are also contained as frames in Supplementary Movie 1. That sequence of TEM images taken in situ at 383 K shows most convincingly that the recrystallization takes place by continuous growth from the rim. Section d has been obtained with the camera installed in our optical tester and it shows the gold frame that is used to find the recrystallized bit after the recrystallization experiments were performed in the fast regime. The white dot delimited by the red circle inside the square is the area illuminated by the probe laser. The same area has been partially melted by the pump laser and isothermally recrystallized at the substrate temperature. The TEM image of section e refers to the annealing of an AgInSbTe sample at 473 K. No structural contrast between the recrystallized bit and the crystalline matrix is visible. When the same experiment (493 K) is performed using Ge₂Sb₂Te₅, the recrystallization is nucleation-dominated and the erased bit has a pronounced contrast in comparison with the crystalline matrix (f).